1. Field of the Invention
The present invention relates to a method of trimming a silicon carbon nitride spacer and a semiconductor structure formed by the aforesaid method.
2. Description of the Prior Art
As semiconductor processes advance to the very deep sub micron level, how to increase the driving current for MOS transistors has become a critical issue in this field.
To attain higher performance of a semiconductor device, attempts have been made to use a strained silicon (Si) layer for increasing the mobility of electrons or holes. Taking advantage of a stressor is a common way to increase the effect of the MOS transistors. Forming a tensile or a compressive stressor on the substrate and using the MOS transistors to change the band structure of silicon can increase mobility of electrons.
A multilayer spacer is often used in the MOS transistors. Therefore, before forming the stressor, the outmost spacer is usually removed so that the stressor is closer to the substrate underneath the gate. After removing the outmost spacer, however, the top surface of the spacer contacting the gate will be higher than the top surface of the gate. Therefore, the stressor deposited on the gate will be uneven.